The present invention relates to an improvement in an electric dust collecting apparatus.
A typical prior art electric dust collector apparatus is illustrated in FIG. 1. In such an apparatus, a low voltage power supply 6 which may merely consist of a commercial A.C. power source provides a relatively low A.C. voltage to back-to-back thyristors 2. The thyristors 2 are controlled by the thyristor control circuit 7 so as to provide a controlled output to reactor 8 which feeds a high voltage transformer 9. The output of the high voltage transformer 9 is rectified by rectifier circuit 10 and the output of the rectifier circuit 10 is supplied to the dust collecting electrodes.
The thyristor control circuit 7 can be operated in either an automatic mode or a manual mode depending on the position of the selector switch 14. In the automatic mode, a spark detection circuit 12 controls a spark frequency control circuit 13 to provide an output to the phase angle circuit 16 which in turn drives the phase control 17. In this manner, the thyristors 2 are automatically controlled so as to vary the D.C. high voltage applied to the dust collecting electrodes 11.
When the selector 14 is in the manual position, a manual operation knob 15 is used to provide the control signal to the phase angle circuit 16. In this fashion, the high D.C. voltage output applied to the dust collecting electrodes 11 may be manually controlled.
However, in such a prior art type of electric dust collector, since dust particles which can be collected are limited, in principle, to those having a specific resistance within the scope of about 10.sup.4 to 10.sup.11 .OMEGA.cm, there is a shortcoming in that the dust collecting performance is greatly degraded for dust particles having a specific resistance within the scope of 10.sup.11 to 10.sup.13 .OMEGA.cm.
Such a degradation of the dust collecting performance of the known electric dust collector is caused by the occurrence of back corona within the dust collector. As a result of an extensive research over many years, the inventor of this invention has confirmed that the occurrence of the back corona phenomena is always associated with a time constant in the range of about 1 second, and hence has invented a novel electric dust collector whose dust collecting apparatus performance would be not degraded or less degraded even for dust particles having a specific resistance within the scope of 10.sup.11 to 10.sup.13 .OMEGA.cm, by making use of this characteristic time constant property of the back corona phenomena. The apparatus of the present invention operates by applying a current intermittently instead of applying a current continuously as is the case with the prior art, so that the current may be interrupted before the occurrence of the back corona phenomena.